As can be seen from an automobile equipped with a seat belt and/or an air bag for ensuring the safety of a passenger, the importance of safety has been recently recognized and safety measures have been taken. As a result, a passenger mortality rate has been declining in recent years. On the other hand, although the development of safety measures for pedestrians is relatively slower than various measures for protecting passengers, its technical development is in progress.
The air bag for protecting a passenger is already known. In addition, there is the following request for the air bag. After the weight on a front passenger seat of a vehicle is detected so as to determine whether a passenger is an adult or a child, the inflation of the air bag is controlled so as to achieve both the protection performance for an adult passenger and the reduction of damages to a child passenger (Japanese Patent Laid-Open Publication No. Hei 15-240632). For such control, it is necessary to provide a load sensor for passenger detection under a passenger seat so as to first detect a weight applied to the passenger seat and then to determine whether the passenger is an adult or a child based on the detected weight. As a result, it is possible to control the inflation of the air bag in accordance with the detected weight. For example, if the detected weight exceeds a predetermined weight, the passenger is determined to be an adult to inflate the air bag with a normal amount of gas. On the other hand, if the detected weight is less than the predetermined weight, the passenger is determined to be a child. In this case, the inflation of the air bag can be controlled so as not to inflate the air bag at all, to inflate it with a reduced amount of gas or the like.
Moreover, as a technique for protecting a pedestrian, a technique for preventing a pedestrian from hitting his or her head against a vehicle hood during a car crash has been developed (Japanese Patent Laid-Open Publication No. Hei 11-028994). According to this technique, in a car crash involving a pedestrian, an impact load is detected so as to open the hood of a vehicle, to inflate an air bag over the hood or the like. In order to actuate such a safety mechanism, it is necessary to detect whether an object colliding against the bumper of a vehicle is a human or not. In this case, for example, if a load sensor is provided for a vehicle bumper, a weight of a colliding object is detected based on a weight detected by the load sensor in the case of collision against the bumper. Then, if the weight is equivalent to a human body weight, the above-mentioned safety mechanism can be actuated.
It is desirable that the load sensor in the above-mentioned techniques be constituted so as to have a high sensitivity to a load from one certain direction but not to react to a load from the other directions. For example, in the case of the load sensor for passenger detection provided under a front passenger seat, it is desirable that only a weight on the passenger seat be detected. If a load from the other directions, for example, from a longitudinal direction of a vehicle or the like is detected, the weight on the passenger seat cannot be precisely measured. Therefore, the control of inflation of the air bag as described above and the like cannot be accurately performed. Moreover, in the case of the load sensor provided for the bumper, for detecting the weight of an object colliding against the bumper, if a load applied in the direction other than the longitudinal direction of the vehicle, for example, a load applied in the vertical direction of the vehicle is detected, the load is detected even by vertical vibration generated when the vehicle is traveling on a bad road. As a result, there is a possibility the above-described safety mechanism may be erroneously actuated to open a hood, to inflate an air bag on the hood, or perform some other safety measure.
To cope with such problems, the above-cited Japanese Patent Laid-Open Publication No. Hei 15-240632 suggests a structure for reducing an unnecessary load applied in a direction orthogonal to a load detection direction. Japanese Patent Laid-Open Publication No. Hei 15-240632 discloses weight measuring equipment wherein a load is applied to a load detecting unit for converting a load into the amount of electricity by a bending strain through a flat spring forming a cantilevered beam. Since the detection sensitivity greatly varies depending on a stiffness ratio of the flat spring to the detecting unit, a length of the flat spring and the like, the components and the assembly of the components require high accuracy, resulting in high cost. Moreover, there is another problem that, for example, a large detection dynamic range cannot be ensured for the strength in the case where a tensile load is applied in the same direction as a load detection direction.
Furthermore, the above-cited Japanese Patent Laid-Open Publication No. Hei 15-240632 discloses a detection device for detecting an impact on a bumper. A load sensor is provided over the entire bumper in Japanese Patent Laid-Open Publication No. Hei 15-240632. However, since the shape and size of bumpers differ for each type of vehicle, the arrangement of a load sensor is required to be designed for each type of vehicle. Moreover, since the area required to place the load sensor is large, the cost is disadvantageously elevated. It is conceivable to provide the load sensor between a bumper reinforcement (hereinafter, also referred to as a bumper R/F) in a vehicle and a pair of front side members so as to avoid the above-mentioned problem. However, the bumper R/F supports a member for absorbing an impact on the front side of a vehicle and, therefore, must have a high stiffness and a large mass. Accordingly, a shearing force in the vertical direction of the vehicle such as the weight of the bumper R/F, vertical vibration and the like are exerted on the load sensor so that the load sensor detects them as disturbance noises. As a result, an impact load cannot be detected with high accuracy.